This invention relates to an improved method for the controlled heating and melting of an end of a semiconductor body and more particularly, to the controlled melting of the end of a polycrystalline semiconductor body and the pulling of a monocrystalline ingot from the molten material.
The electronics industry requires large quantities of monocrystalline semiconductor material, especially silicon, for the fabrication of transistors, diodes, integrated circuits, and the like. The monocrystals are used by the electronics industry in the form of thin, circular wafers or slices a few inches in diameter. The wafers are produced by first growing a monocrystalline ingot or boule which is a long cylindrical, monocrystalline body of approximately the same diameter as that desired for the wafers. The ingot is then sawed into wafers which are lapped and polished to the desired form. Most of the monocrystalline ingots are produced by one of three growth techniques: the Czochralski method, the pedestal method, or the floating zone method. There are inherent limitations in each of these methods as heretofore practiced.
Most crystals in the semiconductor industry are grown by the Czochralski method. In this method high purity polycrystalline silicon is melted in a refractory crucible along with desired amounts of dopant impurities. A seed crystal is dipped into the molten material and then slowly withdrawn to nucleate monocrystalline growth as the molten material cools and freezes. Silicon melts at about 1420.degree. C., and at this temperature any material from which the crucible can be fabricated will adversely dope or contaminate the molten silicon. Monocrystals grown by this method, therefore, have undesirable concentrations of unwanted impurities.
The pedestal method and the floating zone method are similar in that each is a crucible-free growth method. In the floating zone method an elongated polycrystalline body is vertically positioned. The lower end of the polycrystalline body is melted by induction heating from an RF induction heating coil located in proximity to that lower end. The molten material is held in place by surface tension and by the levitating effects of the RF field. The molten zone is caused to move along the length of the polycrystalline body. As the molten zone passes, the material refreezes as a monocrystal, with monocrystalline growth again being nucleated by a seed crystal. Since the molten zone is not supported by a crucible, no undesirable contaminants are introduced into the growing monocrystal. The size of the crystal which can be grown, however, is severely limited. As the diameter of the crystal increases, so also does the volume of molten material. As the volume increases, a point is reached at which the surface tension and levitational forces can no longer support this mass of material and the melt "spills". This results from the fact that the material is heated solely by induction heating from an RF induction coil. Induction heating is a surface phenomenon, with currents being induced in the skin or surface of the polycrystalline body. As the power to the induction coil is increased to melt through the entire cross section of the polycrystalline body, the length of the molten zone also increases, thus increasing the volume of molten material. Consequently, float zone crystals are limited to three or perhaps four inches in diameter. The large crystals have severe radial nonuniformities because of temperature gradients which exist between the center of the semiconductor body and its surface during the crystal growth process. Thus crystals are limited in both size and crystal perfection.
In the pedestal growth method a pool of molten material is maintained on the top end of a polycrystalline body. As in the float zone method, this pool of molten material is maintained by induction heating from an RF induction heating coil. A seed crystal can be dipped into the pool of molten material and then slowly withdrawn, forming a monocrystalline ingot as the material freezes. Like the float zone method, the molten material is held in place by the combined forces of surface tension and levitation from the RF field. The size of the crystal that can be grown by this method and also the perfection of that crystal is thus again severely limited. Increasing the diameter of the crystal requires that a greater mass of material be rendered molten and this poses the same problems of radial temperature gradients and containment of the melt. Crystals grown by the pedestal method have typically been limited to about one and a half to two inches in diameter.
Accordingly it an object of this invention to provide an improved method for the controlled melting of an end portion of a polycrystalline semiconductor body with reduced radial temperature gradients.
It is a further object of this invention to provide an improved method for the crucible free growth of monocrystalline semiconductor bodies.
It is a still further object of this invention to provide a process for the crucible free growth of large diameter, high perfection monocrystalline silicon ingots.